The Android Open Source Project | 9066cfe | 2009-03-03 19:31:44 -0800 | [diff] [blame] | 1 | /* |
| 2 | * Copyright (C) 2008 The Android Open Source Project |
| 3 | * |
| 4 | * Licensed under the Apache License, Version 2.0 (the "License"); |
| 5 | * you may not use this file except in compliance with the License. |
| 6 | * You may obtain a copy of the License at |
| 7 | * |
| 8 | * http://www.apache.org/licenses/LICENSE-2.0 |
| 9 | * |
| 10 | * Unless required by applicable law or agreed to in writing, software |
| 11 | * distributed under the License is distributed on an "AS IS" BASIS, |
| 12 | * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| 13 | * See the License for the specific language governing permissions and |
| 14 | * limitations under the License. |
| 15 | */ |
| 16 | |
| 17 | package android.net; |
| 18 | |
| 19 | import android.os.SystemClock; |
| 20 | import android.util.Config; |
| 21 | import android.util.Log; |
| 22 | |
| 23 | import java.io.IOException; |
| 24 | import java.net.DatagramPacket; |
| 25 | import java.net.DatagramSocket; |
| 26 | import java.net.InetAddress; |
| 27 | |
| 28 | /** |
| 29 | * {@hide} |
| 30 | * |
| 31 | * Simple SNTP client class for retrieving network time. |
| 32 | * |
| 33 | * Sample usage: |
| 34 | * <pre>SntpClient client = new SntpClient(); |
| 35 | * if (client.requestTime("time.foo.com")) { |
| 36 | * long now = client.getNtpTime() + SystemClock.elapsedRealtime() - client.getNtpTimeReference(); |
| 37 | * } |
| 38 | * </pre> |
| 39 | */ |
| 40 | public class SntpClient |
| 41 | { |
| 42 | private static final String TAG = "SntpClient"; |
| 43 | |
| 44 | private static final int REFERENCE_TIME_OFFSET = 16; |
| 45 | private static final int ORIGINATE_TIME_OFFSET = 24; |
| 46 | private static final int RECEIVE_TIME_OFFSET = 32; |
| 47 | private static final int TRANSMIT_TIME_OFFSET = 40; |
| 48 | private static final int NTP_PACKET_SIZE = 48; |
| 49 | |
| 50 | private static final int NTP_PORT = 123; |
| 51 | private static final int NTP_MODE_CLIENT = 3; |
| 52 | private static final int NTP_VERSION = 3; |
| 53 | |
| 54 | // Number of seconds between Jan 1, 1900 and Jan 1, 1970 |
| 55 | // 70 years plus 17 leap days |
| 56 | private static final long OFFSET_1900_TO_1970 = ((365L * 70L) + 17L) * 24L * 60L * 60L; |
| 57 | |
| 58 | // system time computed from NTP server response |
| 59 | private long mNtpTime; |
| 60 | |
| 61 | // value of SystemClock.elapsedRealtime() corresponding to mNtpTime |
| 62 | private long mNtpTimeReference; |
| 63 | |
| 64 | // round trip time in milliseconds |
| 65 | private long mRoundTripTime; |
| 66 | |
| 67 | /** |
| 68 | * Sends an SNTP request to the given host and processes the response. |
| 69 | * |
| 70 | * @param host host name of the server. |
| 71 | * @param timeout network timeout in milliseconds. |
| 72 | * @return true if the transaction was successful. |
| 73 | */ |
| 74 | public boolean requestTime(String host, int timeout) { |
| 75 | try { |
| 76 | DatagramSocket socket = new DatagramSocket(); |
| 77 | socket.setSoTimeout(timeout); |
| 78 | InetAddress address = InetAddress.getByName(host); |
| 79 | byte[] buffer = new byte[NTP_PACKET_SIZE]; |
| 80 | DatagramPacket request = new DatagramPacket(buffer, buffer.length, address, NTP_PORT); |
| 81 | |
Robert Greenwalt | bf7de39 | 2010-04-21 17:09:38 -0700 | [diff] [blame] | 82 | // set mode = 3 (client) and version = 3 |
The Android Open Source Project | 9066cfe | 2009-03-03 19:31:44 -0800 | [diff] [blame] | 83 | // mode is in low 3 bits of first byte |
| 84 | // version is in bits 3-5 of first byte |
| 85 | buffer[0] = NTP_MODE_CLIENT | (NTP_VERSION << 3); |
| 86 | |
| 87 | // get current time and write it to the request packet |
| 88 | long requestTime = System.currentTimeMillis(); |
| 89 | long requestTicks = SystemClock.elapsedRealtime(); |
| 90 | writeTimeStamp(buffer, TRANSMIT_TIME_OFFSET, requestTime); |
| 91 | |
| 92 | socket.send(request); |
Robert Greenwalt | bf7de39 | 2010-04-21 17:09:38 -0700 | [diff] [blame] | 93 | |
The Android Open Source Project | 9066cfe | 2009-03-03 19:31:44 -0800 | [diff] [blame] | 94 | // read the response |
| 95 | DatagramPacket response = new DatagramPacket(buffer, buffer.length); |
| 96 | socket.receive(response); |
| 97 | long responseTicks = SystemClock.elapsedRealtime(); |
| 98 | long responseTime = requestTime + (responseTicks - requestTicks); |
| 99 | socket.close(); |
| 100 | |
| 101 | // extract the results |
| 102 | long originateTime = readTimeStamp(buffer, ORIGINATE_TIME_OFFSET); |
| 103 | long receiveTime = readTimeStamp(buffer, RECEIVE_TIME_OFFSET); |
| 104 | long transmitTime = readTimeStamp(buffer, TRANSMIT_TIME_OFFSET); |
| 105 | long roundTripTime = responseTicks - requestTicks - (transmitTime - receiveTime); |
Robert Greenwalt | 499a121 | 2010-04-15 12:31:55 -0700 | [diff] [blame] | 106 | // receiveTime = originateTime + transit + skew |
| 107 | // responseTime = transmitTime + transit - skew |
| 108 | // clockOffset = ((receiveTime - originateTime) + (transmitTime - responseTime))/2 |
| 109 | // = ((originateTime + transit + skew - originateTime) + |
| 110 | // (transmitTime - (transmitTime + transit - skew)))/2 |
| 111 | // = ((transit + skew) + (transmitTime - transmitTime - transit + skew))/2 |
| 112 | // = (transit + skew - transit + skew)/2 |
| 113 | // = (2 * skew)/2 = skew |
| 114 | long clockOffset = ((receiveTime - originateTime) + (transmitTime - responseTime))/2; |
Robert Greenwalt | bf7de39 | 2010-04-21 17:09:38 -0700 | [diff] [blame] | 115 | // if (Config.LOGD) Log.d(TAG, "round trip: " + roundTripTime + " ms"); |
| 116 | // if (Config.LOGD) Log.d(TAG, "clock offset: " + clockOffset + " ms"); |
The Android Open Source Project | 9066cfe | 2009-03-03 19:31:44 -0800 | [diff] [blame] | 117 | |
Robert Greenwalt | 499a121 | 2010-04-15 12:31:55 -0700 | [diff] [blame] | 118 | // save our results - use the times on this side of the network latency |
| 119 | // (response rather than request time) |
| 120 | mNtpTime = responseTime + clockOffset; |
| 121 | mNtpTimeReference = responseTicks; |
The Android Open Source Project | 9066cfe | 2009-03-03 19:31:44 -0800 | [diff] [blame] | 122 | mRoundTripTime = roundTripTime; |
| 123 | } catch (Exception e) { |
| 124 | if (Config.LOGD) Log.d(TAG, "request time failed: " + e); |
| 125 | return false; |
| 126 | } |
| 127 | |
| 128 | return true; |
| 129 | } |
| 130 | |
| 131 | /** |
| 132 | * Returns the time computed from the NTP transaction. |
| 133 | * |
| 134 | * @return time value computed from NTP server response. |
| 135 | */ |
| 136 | public long getNtpTime() { |
| 137 | return mNtpTime; |
| 138 | } |
| 139 | |
| 140 | /** |
| 141 | * Returns the reference clock value (value of SystemClock.elapsedRealtime()) |
| 142 | * corresponding to the NTP time. |
| 143 | * |
| 144 | * @return reference clock corresponding to the NTP time. |
| 145 | */ |
| 146 | public long getNtpTimeReference() { |
| 147 | return mNtpTimeReference; |
| 148 | } |
| 149 | |
| 150 | /** |
| 151 | * Returns the round trip time of the NTP transaction |
| 152 | * |
| 153 | * @return round trip time in milliseconds. |
| 154 | */ |
| 155 | public long getRoundTripTime() { |
| 156 | return mRoundTripTime; |
| 157 | } |
| 158 | |
| 159 | /** |
| 160 | * Reads an unsigned 32 bit big endian number from the given offset in the buffer. |
| 161 | */ |
| 162 | private long read32(byte[] buffer, int offset) { |
| 163 | byte b0 = buffer[offset]; |
| 164 | byte b1 = buffer[offset+1]; |
| 165 | byte b2 = buffer[offset+2]; |
| 166 | byte b3 = buffer[offset+3]; |
| 167 | |
| 168 | // convert signed bytes to unsigned values |
| 169 | int i0 = ((b0 & 0x80) == 0x80 ? (b0 & 0x7F) + 0x80 : b0); |
| 170 | int i1 = ((b1 & 0x80) == 0x80 ? (b1 & 0x7F) + 0x80 : b1); |
| 171 | int i2 = ((b2 & 0x80) == 0x80 ? (b2 & 0x7F) + 0x80 : b2); |
| 172 | int i3 = ((b3 & 0x80) == 0x80 ? (b3 & 0x7F) + 0x80 : b3); |
| 173 | |
| 174 | return ((long)i0 << 24) + ((long)i1 << 16) + ((long)i2 << 8) + (long)i3; |
| 175 | } |
| 176 | |
| 177 | /** |
| 178 | * Reads the NTP time stamp at the given offset in the buffer and returns |
| 179 | * it as a system time (milliseconds since January 1, 1970). |
| 180 | */ |
| 181 | private long readTimeStamp(byte[] buffer, int offset) { |
| 182 | long seconds = read32(buffer, offset); |
| 183 | long fraction = read32(buffer, offset + 4); |
| 184 | return ((seconds - OFFSET_1900_TO_1970) * 1000) + ((fraction * 1000L) / 0x100000000L); |
| 185 | } |
| 186 | |
| 187 | /** |
| 188 | * Writes system time (milliseconds since January 1, 1970) as an NTP time stamp |
| 189 | * at the given offset in the buffer. |
| 190 | */ |
| 191 | private void writeTimeStamp(byte[] buffer, int offset, long time) { |
| 192 | long seconds = time / 1000L; |
| 193 | long milliseconds = time - seconds * 1000L; |
| 194 | seconds += OFFSET_1900_TO_1970; |
| 195 | |
| 196 | // write seconds in big endian format |
| 197 | buffer[offset++] = (byte)(seconds >> 24); |
| 198 | buffer[offset++] = (byte)(seconds >> 16); |
| 199 | buffer[offset++] = (byte)(seconds >> 8); |
| 200 | buffer[offset++] = (byte)(seconds >> 0); |
| 201 | |
| 202 | long fraction = milliseconds * 0x100000000L / 1000L; |
| 203 | // write fraction in big endian format |
| 204 | buffer[offset++] = (byte)(fraction >> 24); |
| 205 | buffer[offset++] = (byte)(fraction >> 16); |
| 206 | buffer[offset++] = (byte)(fraction >> 8); |
| 207 | // low order bits should be random data |
| 208 | buffer[offset++] = (byte)(Math.random() * 255.0); |
| 209 | } |
| 210 | } |